The present invention relates to an automatic chemical analyzer for specimen tests such as chemical analysis and immunoassay in a field of clinical tests, and in particular, to sampling systems for specimens and reagents in the automatic chemical analyzer. The terms "sampling" and "sample" used herein are employed to mean that liquids (i.e., specimens and/or reagents) are sucked at one position, delivered to another position, and then discharged.
At present, automatic chemical analyzers have been utilised preferably for specimen tests, such as chemical analysis and immunoassay. In general, to relieve a patient of his or her burden against extracting his or her specimen and to reduce reagents to their minimums, the analyzer adopts a microquantity sampling technique of high accuracy by which a minimum specimen sampling quantity of 2 .mu.l per test and a coefficient of variation: CV of 1% or less have been realized. The coefficient of variation: CV is represented by an equation: CV %=(standard deviations of measuring values)/(mean values of measuring values).times.100.
Such high accuracy sampling requires each element part (component) of the analyzer to be highly accurate in operation, which has led to continuing technical development; for example, increases in operating accuracy for syringes and driving systems of sampling pumps, prevention of the pressure loss through the pressure transmitting system ranging from a syringe to a pipetting probe, and quick response of valves.
However, raising the operation accuracy of each component of an automatic chemical analyzer up to almost their limits using such techniques has been attended with a number of technical difficulties, which results primarily in a large increase in manufacturing cost.
Furthermore, there are other drawbacks in conventional-type analyzers. One of them is concerned with their size. Sampling exactly-specified quantities(volumes) of specimens and reagents from a pipetting probe into a reaction cup requires a large number of sensors to be installed in the analyzer, which leads to an unfavorable increase in the entire size of the analyzer.
Another drawback is a problem of contamination of pipetting probes. It is necessary to prevent the tip of a pipetting probe from being contaminated by specimens or reagents for reducing the volume of reagents. To avoid excessive contamination, a device has been used in which only a minimum length of the tip of the pipetting probe is dipped into specimens or reagents. This device necessitates sensors for detecting liquid levels in specimen and reagent cups. Such sensors utilize, for instance, changes in electrical conductance or capacitance.
However, the sensor utilizes changes in electrical conductance, even though it shows favorable sensitivity to conductive liquid solution only, is required to have two electrodes. Therefore, there exists a drawback; liquid solution tends to stick to the ends of the electrodes to remain therebetween. On one hand, the sensor based on capacitance changes, in which one electrode is used, can use a pipetting probe able to work also as the electrode. But, in this sensing system, because of the non-conductiveness of cups from which specimens or reagents are sampled, the sensitivity in detection is decreased and this system can not be easily used for microquantity sampling.
A further drawback common to both the conductivity-depended and capacitance-depended sensors is as follows; when a pipetting probe is also used as an elctrode, the pipetting probe itself should be made of conductive metal. Thus such sensors not be used, when plastic-made disposable pipetting probes (chips) are used.
Furthermore, there is a problem which needs to be solved in conventional analyzers. That is a problem concerning a detection of air suction while being sampled and detecton of jamming at a pipetting probe with impurities in liquid solution such as fibrin in a serum specimen. For detecting Jamming, a semiconductor pressure sensor is disposed in the pipetting probe to detect inner pressure changes therein. However, the semiconductor pressure sensor is a complex construction device and is difficult to deaerate air. In addition, the semiconductor pressure sensor tends to fail, and has a limit in sensitivity; it can not be applied to sampling handling a microquantity of 20 .mu.l or less.